Don't try for love at first sight, please. That's not how it works or how it feels, it's just what movies want to force-feed people ... I mean, I may be biased, but ... it's more like "this person is different, I'm at ease around them, I want to get to know them, my troubles feel smaller when they're near / I feel stronger / happier when I'm with them, I'd like to spend more time with them, I care about what happens to them even though we barely know each other... "
It's not something you can really define as straight up love, not that quickly, and definitely not at that age. It's more important to be earnest at this point than passionate. You really just need to make it clear that Andreas feels that she's special and that Caecillia feels the same way. Some awkward feelings of basic attraction or the mild acknowledgment that she's a pretty girl doesn't hurt either, I guess.
... *rubs temples* I may have just overloaded myself with sappiness.
Edit: As for making her more interesting, you could give her a goal / ambition to give her a little depth. Don't get me wrong, I know she's supposed to be nice and she is. But over all, she just seems like another silly, aimless, sheltered rich-girl. If you recall Lulu's criticism of her character, then maybe you could try to do something to prove her wrong. Alternatively, if you want to make 'being nice' more of a solid and defining trait, I'd suggest showing her doing more. Maybe sneaking out to help the less fortunate? Or perhaps just developing the awareness that caring about something is meaningless in the grand scheme if you don't act.